In forming circuit boards, for use especially in computers, it is necessary to mount various components onto a substrate. Among these components are integrated circuit (I/C) chips. Various techniques have been developed for mounting integrated circuit chips onto circuit boards. One technique is the so-called "direct chip attach" (DCA), in which chips are mounted along with other components directly onto the circuit board or mother board of the computer. While this type of mounting does have certain attractive aspects, nevertheless there are some serious limitations to direct chip attach. These limitations include the thermal mismatch of silicon chips with the circuit board which is typically formed of epoxy fiberglass FR4 technology and which has a significantly different coefficient of thermal expansion (CTE) from that of the integrated circuit chip which typically is made of silicon. Moreover, there is a problem of reworking the circuit board in case of defective chips, which in the extreme case can result in totally scrapping the circuit board at the end of the manufacturing process due to malfunctioning or other problems with a single integrated circuit chip.
For the above and other reasons, one very common technique for attaching integrated circuit chips to circuit boards is by the use of chip carriers, which chip carriers mount the integrated circuit chips, and which carriers in turn are mounted to the circuit board. While this introduces another level of packaging, nevertheless there are certain advantages to this type of chip mounting which, in some instances, make it a more desirable mode of chip attach. In this technique, the integrated circuit chip is secured to a chip carrier either by solder ball connections or wire bonding, and the chip carrier, with one or more chips attached thereto is then attached to the circuit board by various different technologies such as pin-in-hole, solder ball or wire bonding techniques. This has the advantage of allowing the chips to be tested individually or in small groups on individual chip carriers and, if there is a defective or malfunctioning chip, that chip can be easily replaced or, if not replaced, the chip and the carrier can be discarded without discarding or scrapping the entire circuit board at a late state in processing. This technique also allows for greater tolerance to thermal mismatch between the chip and the chip carrier for several reasons.
In one technique, a carrier material can be selected which has a coefficient of thermal expansion between that of the silicon of the integrated circuit chip and the organic epoxy fiberglass material of the circuit board. One such type of carrier is ceramic carriers, which are well known in the art.
Another factor in creating or minimizing the consequences of thermal mismatch between the chip and the circuit board material is that the chip carrier itself is relatively small as compared to the circuit board, and thus the effect of the thermal mismatch vis-a-vis the chip is significantly reduced as compared to situations where there is direct chip attach with a relatively small integrated circuit chip and a relatively large circuit board. Thus, a technique for mounting chips on carriers and carriers to circuit boards has been developed in which the same material is used both for the chip carrier and the circuit board, which in many instances is FR4 epoxy fiberglass material. By using the same material for both the chip carrier and the circuit board, essentially the same technologies can be used for forming both the chip carriers and the circuit board material, thereby reducing the number of technologies necessary to provide a finished product.
Technologies used to form epoxy fiberglass FR4 material into chip carriers include, among other things, the lamination of several layers of the FR4 or organic board together to form the chip carrier and also include photolithographic technologies where photoresist (either positive acting or negative acting) is used either to pattern the material for either subtractive or additive circuitization. In the technologies for manufacturing SCML and MCML chip carriers, because of their configurations, both of these processes can have certain detrimental effects on the manufacturing process which can lead to causing significant problems with the resultant chip carrier to the extent that it is unsatisfactory for its intended use. Thus, it is desirable to eliminate these problems in the FR4 board technology.
Moreover, certain of these problems are encountered not only in FR4 board technology, but in other types of technology such as ceramics for forming chip carriers in which the photoresist process is utilized to form chip carriers, and thus, it would be desirable to eliminate the problems associated with the photoresist process in these other types of carrier technologies.